This invention relates to an exposure apparatus using electron beams. With the known raster scan type exposure apparatus using electron beams, limits are imposed on the width of each scanning in one direction. Therefore, a region in which a pattern is to be drawn (hereinafter referred to as "a pattern region") has to be divided into a plurality of frames. For example, as shown in FIG. 1 representing the prior art, the pattern region 10 is divided into a plurality of band-shaped sections or frames 10-1 to 10-N each having a width equal to the width Xs of electron beam scanning. Scanning is carried out for each frames. Namely, each time electron beam scanning is conducted for a width Xs in the direction X, a table carrying a mask is moved vertically in the direction Y for the width of a scanning line. When the scanning of a frame is brought to an end, the adjacent frame is scanned in the same manner. In this case, the customary practice of attempting to shorten a length of time required to draw a whole electron beam pattern is to scan the first frame 10-1 of FIG. 1, while the table is moved upward. The second frame 10-2 of FIG. 1 is scanned, while the table is moved downward. Thus the frames of the odd number order are scanned while the mask table is moved upward, whereas the frames of the even number order are scanned, while the table is moved downward. The above-mentioned conventional scanning process has to be carried out by changing the order in which serial data for electron beam scanning is to be supplied to an electron beam generator in accordance with the direction in which the table is moved.
Where a desired pattern is drawn, for example, by etching on a semiconductor wafer through a mask, the general practice is to set the mask directly on the semiconductor wafer for, exposure to light, or to dispose an optical system between the mask and semiconductor wafer and expose the semiconductor wafer to light through the mask and optical system. Where a desired pattern is to be drawn on the semiconductor wafer by directly mounting the mask on the semiconductor wafer, then it is demanded to form on the mask a pattern corresponding to the mirror image of a pattern which is to be finally produced on a semiconductor wafer (the term "mirror image" is herein defined to mean a pattern symmetrically disposed with respect to a desired pattern to be formed on the semiconductor wafer). Where a desired pattern is to be defined on a semiconductor wafer, by applying an optical system, it is advantageous to form on the mask a regular pattern, that is, a pattern directly conforming to the desired pattern which is to be finally produced on a semiconductor wafer.
Generally, it is necessary to change the order in which serial scanning data is to be supplied to an electron beam generator in accordance with the direction in which the mask-carrying table is moved, or according as a pattern to be drawn on a mask represents the above-defined regular (directly conforming) image or mirror image (symmetric image) with respect to a pattern finally produced on a semiconductor wafer.